Vanadium-substituted Dawson-type polyoxometalates as versatile electrocatalysts

A selected series of mono- and multi- V-substituted derivatives of Dawson type structure were synthesized and characterized with the aim of exploring their electrochemistry and their electrocatalytic abilities. The focus was placed on the electrochemistry of [P 2 V 2 W 16 O 62 ] 8- as a representative example. The redox processes of its two V-centers were observed in a potential domain well positive of those of the W-centers. They are pH-dependent, the first redox process exhibiting only modest and progressively smaller potential shifts from pH 0 to 4, while the second wave was far more sensitive to acidity changes from pH 0 to 8. In contrast, mono-substituted derivatives display very small or no pH-dependence of the V-wave. Finally, combination of this diversity in the number of V atoms with the presence of As or P as the central heteroatom in these tungstic and molybdo-tungstic structures modulates substantially the apparent formal potentials that span the range from + 569 mV to + 122 mV vs SCE at pH 7. This leaves considerable flexibility in the choice of POMs for electrocatalytic purposes. The homogeneous oxidation and the electrocatalytic reduction of nitrite and the electrocatalytic oxidation of NAD(P)H by an appropriate selection of these V-substituted anions were studied.